A device that uses light to detect particles in a fluid, where two mirrors are used to reflect deflected light back to a light detecting device and one of the mirrors is formed from the housing of the light-detecting device.
Because of the small size of semiconductors when manufacturing semiconductors it is critical that particles not be permitted to contaminate the process. Particles as small as 1 μm and less can contaminate the process. The first generation of semiconductor manufacturing plants were built with the so-called open ballroom concept. Here an attempt to keep the entire plant free of particles was made. Each successive generation of manufacturing plant design has made the clean space where particles are eliminated smaller and smaller. The latest design of manufacturing plants has what are called mini-environments. These environments are just big enough to contain the tools that work on the silicon wafers. Silicon wafers are transported from tool to tool in containers that attach to the tools in a process that is analogous to two space ships docking. The goal is to eliminate the possibility of particles entering into either the wafer's transport pod or the tool's mini-environment.
There is a need to constantly monitor the tool's mini-environment to prevent expensive silicon wafers from being contaminated by particles. The tool's mini-environment is essentially no larger then necessary to contain the tool. Because adding space to these mini-environments is expensive and semiconductors are continuously getting smaller and smaller, there is a need for particle detectors to remain small yet detect smaller and smaller particles.
A basic design for a particle detector is illustrated in FIG. 1. The components are shown suspended in space to facilitate understanding their relationship with one another. The three basic components are a light source (10), a light detection device (20), and a source of particle flow (30). Three axes (1,2,3) are illustrated that are normal to each of the three basic components. These axes can be normal to one another as in Euclidian geometry, but they need not be. Arrow (31) indicates the direction of particle flow. Arrow (11) indicates the direction of the light (15). In most relevant art, the light source (10) is a laser, the light detection device (20) is a photodiode, and the particle flow (30) is achieved with a conventional pump (not shown) pushing or pulling the particles along axis 3.
In the FIGS. 1, 3, and 5, the light rays (15) being emitted by the light source (10) are represented as lines as is the custom with ray tracing. And, the particles (35) are represented by small circles. The sizes of the particles (35) are exaggerated for illustrative purposes. The particles (35) come out of the particle source (30) with some velocity and intersect with the light rays (15). The view volume (40) corresponds to the three dimensional intersection of the particle flow (35) and the light rays (15). The view volume (40) is represented by a sphere, as is the custom in discussing particle detectors.
An example of how particles (35) are detected will now be explained. A simple model of light (15) will be used so that ray tracing can be used in order to illustrate the basic method of operation. The particle flow (35) goes through the view volume (40), and light rays (15) strike the particles (35). First light ray 101, third light ray 103, fourth light ray 104, and fifth light ray 105 have collided with particles (35) in FIG. 1 and are scattered. For purposes of discussion, these four light rays (101, 103, 104, 105) will be called scattered light rays (101, 103, 104, 105). Second light ray 102 does not collide with a particle (35). Most light (15) would not collide with a particle (35) and would merely pass through the view volume (40) and proceed down the axis 1. In most relevant art, the light rays (15) will enter a conventional light trap (not shown). Fourth light ray (104) will collide with the light detection device (20); however, the first light ray (101), the third light ray (103), and 105 will not collide with the light detection device (20).
The fourth light ray (104) striking the light detector (20) is used to define the existence of the struck particle (33) and based on the signal strength, the size of the particle (33). The ability to accurately count and size particles is based on the signal strength above the background noise of the system. The greater the signal to noise ratio, the smaller the particle that can be detected and sized. The noise of the system is caused by stray light striking the light detection device (20).
The more of the light rays (101, 103, 104, 105) that were scattered in the viewing sphere that are collected by striking the light detection device (20) the more sensitive the particle detector will be. Relevant art is concerned with increasing the particle detector's ability to record the scattered light rays (101, 103, 104, 105) by redirecting the scattered light rays (101, 103, 105) that would miss the light detection device (20). This redirection is accomplished with mirrors.
The basic principle is that the more of the scattered light rays (101, 103, 104, 105) that can be detected by the light detection device (20), then the more sensitive the particle detector will be, and the less power the particle detector (20) will need to consume for a given sensitivity by the laser (10).
FIG. 2 is a top view of an improved particle detector. A mirror (50) has been added to the basic design and placed on the opposite side from the light detection device (20). The particle flow (35) is not shown in FIG. 2, but is coming into the paper at a right angle. The light rays (15) travel along axis 1 and collide with particles inside the view volume (40) and are scattered. The mirror (50) reflects tenth light ray (110), eleventh light ray (111), twelfth light ray (112), and thirteenth light ray (113) that would have been missed by the light detection device (20) onto the light detection device (20). This is accomplished by making the mirror (50) an ellipsoidal mirror (50) with one focal point (51) at the view volume (40) and the second focal point (52) at the light detection device (20). This arrangement relies on a basic property of ellipsoidal mirrors (50), where a light ray (15) that originates at one focal point (51) will be reflected by the ellipsoidal mirror to the other focal point (52).
In FIG. 2, sixteenth light ray (116), and seventeenth light ray (117) are scattered by striking particle (35) and go directly to the light detection device (20). Not shown is the light that is scattered and still not collected, because it does not hit either the mirror (50) or the light detection device (20). The mirror (50) improves the signal strength at the light detector (20) by focusing the scattered light rays, the tenth light ray (110), the eleventh light ray (111), the twelfth light ray (112), and the thirteenth light ray (113), from the opposite side of the light detection device (20). U.S. Pat. No. 4,422,761 issued to Frommer on Dec. 17, 1983, illustrates this basic one mirror design, where the viewing volume (40) is at one of the foci (51, 52) of an ellipsoidal mirror (50).
The limitation of the one ellipsoidal mirror (50) design illustrated in FIG. 2 by Frommer is that the fourteenth light ray (114) and the fifteenth light ray (115) are scattered by the particles (35), but are not captured by the detector (20). This reduces the sensitivity of the particle sensor. One possible solution to this problem is to increase the size of the light detection device (20); however, any larger light detection device (20) is very expensive and requires custom manufacturing.
The relevant art deals with methods of reflecting the fourteenth light ray (114) and the fifteenth light ray (115), the missed scattered light rays, with a second mirror opposite the first mirror. U.S. Pat. No. 5,767,967 issued to Yufa on Jun. 16, 1998, illustrates an arrangement of two opposing ellipsoidal mirrors, where as in FIG. 2, the viewing sphere (40) is at the focus of an ellipsoidal mirror. However, the device requires a second ellipsoidal mirror that is both expensive and requires space behind the light detection device (20). Further, the device requires a spherical light detection device (20), which would be very expensive to manufacture.
The added space for the second ellipsoidal mirror makes it difficult to construct a small particle detector. Small particle detectors are needed for making hand-held devices and for fitting particle detectors in larger devices such as silicon wafer tools. Further, the added space needed behind the light detection device (20) by the second ellipsoidal mirror prevents upgrading existing single mirror detectors as there is not enough room in the particle detector's housing to accommodate this extra room.
Thus a need has been established for a particle detector based on an ellipsoidal mirror (50) with a viewing volume (40) at one of its foci that has a second mirror that is not expensive to manufacture and does not require an increase in the volume required to house the particle detector. And further, that does not require a special light detection device (20).